Planographic printing plate and treatment thereof



Patented July 10, 1951 PLAN OGRAPHIC PRINTING PLATE AND TREATMENTTHEREOF Frederick H.

Frost, Portland, MaineQassignor to Warren Company, Boston, Mass., acorporation of Massachusetts No Drawing. Application December 12, 1947,

Serial No. 791,447

Claims. (Cl. 101149.2)

This invention relates to plates for planographic printing and to thepreparation and treatment thereof.

Planographic or lithographic printing depends upon the immiscibility ofthe wet-out liquid and the lithographic printing ink. To print bylithography there is ailixed to a suitable surface or plate awater-repellent image, usually greasy, waxy, or resinous in nature. Tothe plate so imaged is then applied wet-out solution, usually a slightlyacidified aqueous solution of glycerine or similar substance. Thewet-out solution wets all portions of the plate not already covered .byimaging material. An inking-roll coated with lithographic printing inknow passed over the plate leaves a film of ink upon the imaged areas butleaves no ink on the unimaged areas which carry a film of theink-repellent wet-out solution. The inked plate when brought intocontact with another ink-receptive surface transfers thereto the ink ina pattern reverse to that on the plate. The surface so printed upon maybe a paper sheet, but in most cases is an oifset blanket which in turntransfers the-image as av positive to a paper sheet which is the finalprinted matter. The plate is repeatedly dampened, inked and printed fromuntil the desired number of prints have been made.

In recent years very substantial improvements have been made in theproduction of paper-base plates for lithographic or planographicprinting. In general terms, such plates may be said to comprise a paperbase, with or without wetstrength properties, and on at least onesurface of said base a coating comprising hydrophilic film-formingmaterial containing dispersed therein some filling material such asclay, blanc flxe, or the like. In some cases a. lightweight wash coatingof substantially clear film-former, i. e. free of filling material, isapplied over the before mentioned filled coatingin order to preventexcessive penetration by wet-out liquid, i. e. to afford improvedhold-up of water or wet-out liquid. In the latter case, however, duecare must be exercised to be sure that the top coating is not soimpenetrable or non-porous that imaging material cannot be securelyafllxed thereto.

Obviously it is essential that the image shall stick to the platetenaciously. At the same time the unimaged areas must be very easily andcompletely wet by water or aqueous wet-out or dampening solution.Ideally the unimaged surface should be such that while it tenaciouslyholds water applied thereto it will permit no penetration of water. Ithas been found, however, that as hydrophilic surfaces improve in abilityto 2 withstand penetration by water they also tend to I become less andless adhesive to imaging mateimage, and comprises a metal compoundcapable of reacting with a later to be applied reagent to form arelatively water-insoluble metal compound, the image is afilxed to saidsurface, and afterapplication of the image the surface is furthertreated with the reagent to increase the water-impenetrability of saidsurface in its unimaged areas. It appears probable that the resultingwater-impermability of the treated hydrophilic surface is the result ofthe formation of a relatively insoluble substance which acts to fill orclose.the pores or interstices of the hydropholic surface, so that waterpenetration is materially decreased without impairing the wettability ofthe surface and a film of water, i. e. wet-out liquid is held on thesurface to repel printing ink and prevent the non-image areas fromaccepting ink.

In general in practicing the invention, it is preferred to have themetal salt in the hydrophilic coating which is to react with the laterto be applied treating solution, at or near the surface of thehydrophilic coating. Soluble metal salt may be applied in aqueoussolution to the surface of the hydrophilic coating and may be driedthereon whereupon the salt presumably deposits in the surface pores ofthe coating. In practice, however, instead of applying the solution ofthe metal salt in a separate step its application may usually becombined with some other step in the preparation of the plate. If, forexample, the plate is of the type that has a wash coat of hydrophilicsubstance over an underlying filled hydrophilic coating, the metal saltadvantageously may be included as an added ingredient in the said washcoat. If no wash coat is used, the metal salt may be included in thefilled coating, though. this procedure may be somewhat less economicalin consumption of the metal salt since in this case the latter willpermeate the entire thickness of the coating layer rather than beingconfined at or near the top surface only as when either of the twoformer procedures is used.

Naturally the metal salt must be chosen with regard to the otherconstituents of the coating. That is, the metal salt must not becompletely consumed by reaction with any ingredient of the coating, orif there is reaction between the metal alt and another ingredientof thecoating sufficient of the metal salt must be used so that an excess orresidue thereof remains to react with the reagent subsequently applied.Preferably the metal salt remains trapped in the pores of thehydrophilic surface without crystallizing to an appreciable extent ontop of the exposed surface. Excessive crystallization of metal salt ontop of the surface may make it difllcult or impossible to affix an imagesecurely to the surface. In general such excessive crystallization ontop of the surface can be avoided by limiting the quantity of solutionof metal salt applied and controlling the conditions under which thematerial is dried. In a case when excessive metal salt for any reason isleft exposed on the surface the undesired excess can be removed bysubjecting the surface or plate to rubbing or brushing as disclosed inthe pending applications of Frederick H. Frost, Serial Nos. 691,654,filed August 19, 1946 (now abandoned), and 760,939, filed July 14, 1947,which matured into Patent No. 2,534,588, December 19, 1950.

The plates disclosed in pending applications of Stephen V. Worthen,Serial No. 747,138, filed May 9, 1947 (now abandoned), and Serial No.758,215, filed June 30, 1947 (now abandoned), said plates having ahydrophilic colloid surface containing a divalent metal salt areexamples of plates of the type described above, i. e. having ahydrophilic colloid surface comprising a metal salt capable of reactingwith a later applied reagent to form a pore filling or cloggingrelatively water-insoluble metal compound.

The hydrophilic surface containing the metal salt, rubbed or brushed ifdesired, is then marked with a lithographic imaging material, as byprinting, typewriting, pen and ink, wax or resinous crayon, or the like.

After the image is applied the imaged surface is washed over with areagent which will react with the metal salt already present in thepores of the hydrophilic printing surface to form an insoluble metalcompound in the pores. The insoluble compound so deposited leaves thesurface wettable by water but, I believe, to a large extent blocks thepores of the surface and prevents or at least greatly diminishespenetration of water or wet-out solution into or through the hydrophiliccoated surface. The surface can then be kept wet with a minimum ofwet-out solution, and while so wet is very resistant to soiling orscumming by printing ink when used as a plate on a lithographic oroffset press.

A preferred class of insoluble metal compounds to form in the surfacepores for enhancement of water hold-up and impenetrability comprises thevarious relativel insoluble metal phosphates. Many of these salts, inspite of their relative insolubility, crystallize with water in themolecule, and perhaps partly for this reason they are very easilywettable by water.

In order to form such desirable relatively insoluble metal phosphates inthe surface pores of the unimaged areas of the plate the metal saltincluded in the hydrophilic surface as previously described may be asoluble salt of any metal which forms a relatively insoluble phosphate.Then after the plate has been imaged the surface is further treated witha solution containing phosphate ions, either simple or complex- 4 By thereaction of the phosphate ion with the soluble metal salt relativelyinsoluble metal phosphate is formed in situ in the pores of the coating.

The phosphate ion may be supplied by a solution of any soluble phosphateor polyphosphate. Suitable salts include monoand iii-ammoniumphosphates, mono-, di-, and tri-sodium phosphates, sodiumpyrophosphates, sodium hexametaphospbate, etc. For best results enoughphosphate ion should be available to react with substantially all of thesoluble metal salt available for reaction in the hydrophilic coatedsurface.

The application of phosphate ion can very conveniently be made at thetime the plate is treated with wet-out solution preliminarily to runningthe plate on a printing press. In other words, the phosphate ion may beincluded in the wet-out solution. Since it is well-known that to avoidemulsification of lithographic ink it is advisable to keep all dampeningand wet-out solutions non-alkaline, or preferably slightly acid, it isadvantageous to use a phosphate which itself gives a non-alkalinesolution, such as mono-ammonium phosphate. or to buffer the phosphatesolution to yield a pH value of not over 7.0. A pH range of from about 3to about 7 has been found to be satisfactory.

As has been previously indicated the soluble metallic ion used to reactwith the phosphate ion to cause precipitation of the metal compoundwhich leaves the hydrophilic surface waterwettable butwater-impenetrable may be an ion of an metallic element which readilyforms a relatively insoluble phosphate. Operable compounds includesoluble salts of practically all metals except the alkali metals, i. e.the divalent and multivalent metals. It has been found that whenproteins are present in the hydrophilic coating of the plate the agingqualities thereof may be adversely affected by action of salts ofcertain metals, such as aluminum, chromic, ferric, manganic salts andthe like. In such cases it is preferable to use soluble salts ofdivalent metals as the soluble material includedin the hydrophiliccoating. If the hydrophilic coating is such that its aging propertiesare not harmed by metal salts other than those of divalent metals, e. g.the trivalent metals, such other salts of course may be used in thecoating. Or to avoid all possibility of deleterious effects from agingthe hydrophilic coating in contact with a metal salt the procedure maybe altered as follows: a soluble phosphate may be included in thehydrophilic coating and then after the plate has been imaged it may bewashed with a solution of an appropriate metal salt capable of reactingwith the soluble phosphate to form a relatively insoluble compound. Aninsoluble phosphate will thus be formed in the pores to givesubstantially the same eifect as the alternate procedure. In thisaltered procedure the soluble phosphate is substituted for the metalsalt and the metal salt is substituted for the soluble phosphate in theprocedure described above.

Other insoluble compounds besides metal phosphates may be precipitatedin the coating pores for the same purpose, e. g. metal oxalates andborates. Or compounds like barium sulfate may be deposited as byreaction of barium chloride and sodium sulfate, either of which may beused in the washing solution if the other is present in the hydrophiliccoating.

The result of the practice of the invention is A auaum.

a, printing plate having a paper or similar base with on at least oneside a hydrophilic coating,

the coated surface having securely afllxed there- Example 1 A sheet ofpaper body-stock having wetstrength properties derived from inclusion ofmelamine-formaldehyde resin in conventional manner, was coated on oneside with about 5 pounds dry weight per'thousand sq. ft. of surface, ofa coating containing 100 parts of clay and 18 parts of casein dispersedby ammonia. The coated paper was dried and then washed over with aaqueous solution of zinc acetate. The so-treated sheet was dried,calendered, brushed, and typed on by a typewriter. The typed sheet wasswabbed over with a pad of cotton wet with 4% aqueous solution ofdi-ammonium phosphate and then was used as a plate in an offsetduplicator where it proved to be very effective in holding water on itssurface and very resistant to scum formation. Zinc phosphate orzinc-ammonium phosphate presumably was formed in the pores of the caseincoating.

Example 2 A wet-strength paper body-stock was coated on each side withabout 5 pounds dry weight per thousand sq. ft. of a coating containingclay 100 parts, ammonium caseinate 18 parts, and di- 'methylolurea 1parts, and dried. This baseton pad wet with 3% aqueous solution of mono-Example 3 A clay and casein base-coated sheet like that used in Example2 was wash-coated on one side with an aqueous solution containing 1 ofsodium alginate and 10% of diammonium phosphate.

This was dried and calendered, an image was fixed on the treated side,and the imaged sheet was swabbed with a 3% aqueous solution of zincformate. Zinc-ammonium phosphate was prekept its unimaged areassatisfactorily free from scum during its useful life.

Example 4 The clay-casein base-coated sheet of Example 2 was treatedwith an aqueous solution of 2% 'of gum arabic and 10% of calciumacetate, dried,

calendered, brushed, imaged and then swabbed with 4% ammonium oxalatesolution to form insoluble calcium oxalate and used as a printing platewith satisfactory results.

sumably formed. when used as a plate this sheet 6 Example 5 Theclay-casein base-coated sheet of Example 2 was treated with an. aqueoussolution containing 0.5% of carboxymethyl cellulose and 10% of ammoniumsulfate, dried, calendered, brushed, imaged, swabbed with 3% solution ofbarium chloride to form insoluble barium sulfate and used as a printingplate with satisfactory results.

Example 6 Example 2 was repeated but a 4% solution of ammonium boratewas substituted for the ammonium phosphate. Insoluble zinc boratepresumably was formed. The plate gave satisfactory printing results.

Example 7 Example 2 was repeated but the ammonium Example 8' Theclay-casein coated base of Example 2 was washed with an aqueous mixturecontaining 0.2% of guar gum and 12% of lead acetate, dried, calendered,brushed, imaged, swabbed with 5% solution of ammonium chromate to forminsoluble lead chromate and the resulting plate printed withsatisfactory results.

In practice of the invention as illustrated in the foregoing examples Ihave found it to be preferably generallyto first incorporate the solublemetal salt in the hydrophilic paper coating and to then, after imagingapply the reagent which is to react with the metal salt. My extensiveexperience indicates that zinc acetate and zinc formate are generallypreferable to the metal salts of strong mineral acids. My presentexperience indicates that mono-ammonium phosphate is the preferredreagent and that a 3% solution thereof is preferred although strongersolutions thereof up to 10% appear to give identical results. A 2%solution does not appear to be quite as effective when applied in a thincoating with a cotton swab as a 3% solution. I have found it to bepreferable generally to employ a final treating solution, whether themetal salt solution or the solution of a compound the anion of which iscapable of forming an insoluble compound with the metal which is neitheralkaline nor too strongly acid but has a pH value within the range from3 to 7.

The invention has been described and illustrated above with reference toits preferred application for improving the resistance of the un-.

imaged portion of the plate to penetration by wetout liquid but thedescribed treatment of the plate may be useful if the imaging step ismade to follow the treatment of the plate with the metal salt and thereagent. Under certain circumstances an image applied subsequently maybe suiliciently permanent and the unimaged plates are also useful forthe production of photolithographic plates by a subsequent applicationof a photo-sensitive coating and for other purposes.

I claim:

1. Process for the production of a planographic printing plate having ahydrophilic printing surface which comprises incorporating into saidsurface prior to imaging a water-soluble compound capable of reactingwith a water-soluble salt of a metal having a valence greater than 1 todeposit a water-insoluble compound of said metal.

2. Process as defined in claim 1 in which the water-soluble compound isa member of the group consisting of phosphates, oxalates, sulfates, b0-rates, citrates and chromates.

3. Process as defined in claim 1 in which the surface is imaged and asolution of a water-soluble salt of a metal having a valence greaterthan 1 and capable of reacting with the water-soluble compound to form awater-insoluble compound of said metal is applied to the imaged surface.

4. A planographic printing plate having a hydrophilic colloid printingsurface which, prior to imaging, contains a water-soluble compoundcapable of reacting with a water-soluble salt of a metal having avalence greater than 1 to form 1;

an insoluble compound of said metal.

5. An imaged planographic printing plate having a hydrophilic colloidprinting surface, said 8 surface containing in the imaged areas thereofonly a water-soluble compound capable of react- .ing with awater-soluble salt of a metal having a valence greater than 1 to form aninsoluble compound of said metal, andin the unimaged areas saidinsoluble compound of said metal.

FREDERICK H. FROST.

REFERENCES CITED The following references are of record the tile 0! thispatent:

UNITED STATES PATENTS Number Name Date 2,003,268 Wescott May 28, 19352,154,219 Shepherd Apr. 11, 1939 2,373,287 Bassist. Apr. 10, 1945

